High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been avilable for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. Nos. 2,465,319 and in Pengilly, 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
With the development of molecular weight control, use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
A useful family of such compositions are those which are glass-reinforced, e.g., with from about 10 to about 40% of filamentous glass, based on the weight of glass and polyester component.
However, there is a need to improve high voltage dielectric breakdown resistance. For example, addition of 30% by weight of glass filaments to poly(1,4-butylene terephthalate) and molding the composition into rotors for automotive ignition system distributors, provides test pieces which show 50% failure in 100 hrs. when tested continuously at 30,000 volts. Since peak voltages in automobile distributors range from about 25,000 to 50,000 the breakdowns sometimes are noticed after only 35,000 to 50,000 miles of driving.
It has now been discovered that the use of aluminum silicate as a filler alone or in combination with glass fibers measurably improves the long term high voltage dielectric breakdown resistance of polyester resins. As an illustration, a poly(1,4-butylene terephthalate) composition containing 15% by weight of anhydrous, calcined, aluminum silicate and 20% by weight of glass fibers was found to have three times the expected life in comparison with 30% glass reinforced poly(1,4-butylene terephthalate). Tests conducted on molded rotors at 30,000 volts, continuously, showed that the aluminum silicate-containing composition had a 50% failure rate of 300 hours, whereas the 50% failure rate was only 100 hours with the aforementioned composition with 30% glass, but no aluminum silicate.